1. Field of the Invention
This invention relates to a method for producing a high-strength cement article. More particularly, this invention relates to a method for producing a high-strength cement article wherein the porosity and pore size thereof are decreased by the steps of molding by employing specified kneading means with an organic binder, high-temperature firing, and rehydration.
2. Prior Art
Conventional inorganic cement articles are represented by concrete articles, asbestos cement plates, cement roofing tiles, calcium silicate plates, terrazzo blocks, etc. These articles are produced, for example, by adding aggregate, water, and as necessary reinforcing fiber materials, etc. to inorganic cement, mixing them together using a usual mixer, and then either packing the resulting mixture into a mold for molding or molding the mixture to a large mass in a similar manner and cutting the uncured mass to suitable size. Such packing methods include manual packing method and mechanical packing method. Representative molding methods which can be used include pressure molding, vibration molding, pressure-vibration molding, centrifugal molding, extrusion molding, screen molding, etc. In these methods, it is desirable to expel air bubbles and excess water to pack the materials as dense as possible. Thus, such molded articles can be sometimes removed soon after molding operation. Curing of cement is conducted preferably by using water curing, steam curing or autoclave curing to produce cured cement articles. These articles, however, contain a considerable amount of pores and thus are insufficient in strength; they have a bending strength of approximately 60 to 100 kgf/sq.cm which is far lower than the theoretical strength of cement (bending strength of approximately 400 kgf/sq.cm). Even when a reinforcing fiber material is incorporated therein, the strength is approximately 250 kgf/sq.cm. Incidentally, the bending strength is determined in accordance with JIS K7203.
It has been known to increase the strength of a cement article by blending cement materials with an organic synthetic resin, although there remain serious problems such as heat-resistance, flame-resistance, weather-resistance and water-resistance. It is also known that by blending cement materials especially with a water-soluble synthetic resin and curing the resulting blend, there can be obtained a cement product having a bending strength approximately as large as the theoretical value of cement or more. In this case, however, the water resistance or the durability in a wet state is markedly deteriorated. Moreover, there are problems with respect to deterioration due to heat, ultraviolet rays, etc., and to flame resistance. Thus, uses of such cement products are extremely restricted.
The present inventors previously developed a method of making cement articles having good mechanical strength. The method comprises molding a cement concrete mixture, firing the resulting molded article at a high temperature either prior to its hydration curing or after preliminary hydration curing, and then sufficiently hydrating the fired article (cf. U.S. Pat. No. 4,407,769 or Japanese Patent Publication No. 48464/81). The cement article obtained in this method, however, has a bending strength of as low as about 150 kgf/sq.cm or less. Even when reinforcing fiber materials are incorporated therein, the resulting cement article has about 300 kgf/sq.cm or less. Incidentally, it was possible in this method to add to the cement mixture a minimum effective amount of a water-releasing agent or less than about 0.1 part by weight of a paste agent (e.g. CMC and the like) to increase the moldability and the like of the above mentioned concrete molded article. However, it was naturally disadvantageous in view of the strength of the product to add such agent, because such organic substance was burned away to increase voids during the firing step.